There are alternators, which are one type of rotary electric machines, equipped with a power converter for performing, using upper- and lower-arm (high- and low-side) switching elements for multiphase armature windings, synchronous rectification of a phase voltage of one-phase armature winding in the multiphase armature windings, an example of which is disclosed in Japanese Patent Application Publication No. 2010-110176. The power converter of the alternator disclosed in the Patent Publication measures a time “Count” taken from a turnoff of an upper-arm switching element to when a corresponding phase voltage, which exceeded a fourth threshold V4, drops a third threshold V3, thus monitoring an off timing of the upper-arm switching element.